Drop MODEL_SETTINGS from example charms and Charmcraft profiles

In `tox.ini` for our example charms we have

```
[testenv]
...
pass_env =
PYTHONPATH
CHARM_BUILD_DIR
MODEL_SETTINGS
```

I assume that `MODEL_SETTINGS` is there to make it easier to pass model config settings to Juju for integration tests. But we don't document this anywhere or read `MODEL_SETTINGS` in Jubilant etc. So the charmer would have to figure out how to wire up everything themselves.

Is it really a useful placeholder? How about we drop it from the example charms and Charmcraft profiles.

Search the example charms' tox.ini files and the Charmcraft profiles for MODEL_SETTINGS, then confirm how each occurrence is used. Remove the placeholder entries from those configurations and verify that no remaining example or profile references depend on them.

Written by the indexing model from the issue text.